The present invention relates to an apparatus for, by application of energy of distributed levels, recording an image having density or saturation (tone) each corresponding to a respective one of the energy levels, on a pixel basis.
Today, a multi-color heat-sensitive recording apparatus is available which records images in a plurality of colors by use of a multi-color heat-sensitive recording paper, which is provided with a laminate of a plurality of coloring layers thereon. For example, when a multi-color heat-sensitive recording apparatus capable of recording images in red as well as in black and white is applied to a plotter of a facsimile terminal, a document which is partly underlined or otherwise marked in red for emphasis or which includes characters written in red for the same purpose may be sent to a remote terminal to enhance effective image data transmission. Another possible application of the above described type of recording apparatus is to a graphic input terminal, or so-called electronic blackboard, which should allow one to record and output minutes and others as effectively as possible.
A recording paper for use with such a black, white and red type heat-sensitive recording apparatus includes a paper base, and three different layers which are sequentially laminated on the paper base, i.e., a red coloring layer which colors in red, a black erasing layer which erases black, a black coloring layer which colors in black, and a protective layer. When a heating resistor having a contact surface which is dimensioned to define a single pixel is brought into contact with the protective layer and, then, heated, the three consecutive layers are selectively effected depending upon the amount of heat applied to the laminate. Specifically, when a sufficient amount of heat is applied to the protective layer and black coloring layer, the black coloring layer colors over its limited area which corresponds to the pixel so as to record a black pixel; when a greater amount of heat is applied, the black erasing layer is effected to erase the black pixel; and when a still greater amount of heat is applied, the red coloring layer colors to record a red pixel. It is to be noted that the amount of heat (energy) required for a red pixel to be formed is multiple times, e.g., about three times greater than that required for a black pixel to be formed.
A multi-color heat-sensitive recording apparatus of the kind using a heat-sensitive paper as stated above is only illustrative, and other various kinds of such recording apparatuses are known in the imaging art, e.g., a one in which the area of a pixel is dependent upon the energy level.
In the above described kind of recording apparatus, a record head is implemented with a thermal head having a predetermined number of heating elements, or heating resistors, which are arranged in an array and each has a size corresponding to that of a pixel. The heating elements are driven on the basis of one line of image while, at the same time, the heat-sensitive recording paper is moved relative to and perpendicular to the array of heating elements, whereby an image is recorded in the heat-sensitive paper. Assuming that the recording density available with the recording apparatus is eight dots per millimeter and the recording width is 216 millimeters, which is the width of an A4 format, the thermal head needs to be furnished with 1728 heating resistors in total. Usually, the thermal head is produced as a single device in which the wirings to the individual heating resistors are implemented by a printing process.
In case that the thermal head with the above configuration is used and driven at a time, a power source having a substantial capacity is needed to drive, at maximum, the 1728 heating resistors simultaneously. For this reason and, because it is impracticable to supply such a large current at a given time due to the inherent construction of the thermal head, it is the usual practice to divide the thermal head into a plurality of blocks and sequentially supply current to the discrete blocks. This arrangement cuts down the required capacity of a power source as well as the current which should be supplied at a time.
Usually, the service life of a thermal head is exponentially reduced as the amount of heat generated by heating resistors elements per unit time and the duration of continuous drive, i.e., the pulse width of a strobe signal are increased. Hence, should the thermal head be driven continuously over the period of time assigned to a red image and which is three times longer than the period of time assigned to black and white images, it would become unoperable soon. For this reason, it is impossible for the thermal head to be driven directly over such a long continuous period of time. In the light of this, it has been customary to record a red image by repetitively tracing the same line for a duration which would ensure a certain life of the thermal head each time and, thereby, applying a required amount of heat.
In the above situation, a prior art multi-color heat-sensitive recording apparatus consumes a considerable period of time to record a red image and, therefore, cannot be readily put to practical use.